ag (n.)

abbreviation of agriculture, attested from 1839s (in Secretary of Ag., etc.); by 1880s in reference to college courses, American English.

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Definitions of ag from WordNet

Ag (n.)
a soft white precious univalent metallic element having the highest electrical and thermal conductivity of any metal; occurs in argentite and in free form; used in coins and jewelry and tableware and photography;
Synonyms: silver / atomic number "